The present invention relates to a method of starting a hydraulic machine such as a variable speed pump turbine or a variable speed pump.
Japanese Pre-examination Patent Publication No. 126675/1981 discloses a method in which the speed of a pump turbine in a pumping-up power plant is controlled for the purpose of improving the efficiency of the plant. This literature, however, does not disclose any method which would reduce the impact on the electric power line which is caused when the pump turbine is started in the pumping mode. FIG. 3 shows the starting characteristics of a variable speed pump turbine with movable guide vanes, as observed when the pump turbine is started in the pumping mode in accordance with a conventional starting method. More specifically, in FIG. 3, the axis of abscissa represents the time, while the axis of ordinate represents the opening degree of the guide vanes, speed of the pump turbine and the pump input. As will be understood from this Figure, the conventional starting is basically such that the pump is started and operated at a constant speed with the guide vanes fully closed and, when the delivery pressure has been increased to a level above the water head of the pond applied to the delivery side of the pump turbine, the guide vanes are opened thus commencing the pumping up of the water. As will be seen from FIG. 3, the speed of the pump turbine is maintained from the time of start-up till the time of rated operation, but the pump input is drastically increased as soon as the guide vanes start to open.
FIG. 4 shows the operation characteristics of a typical pump turbine with guide vanes, as observed when the pump turbine is operating at a constant speed No. More specifically, in this Figure, the axis of abscissa represents the flow rate, while the axis of ordinate represents the pump input and the delivery head. It will be seen that the pump input when the guide vanes are fully closed, i.e., when the flow rate is zero, is not negligible with respect to the rated pump input. It will be seen also that the pump input is ruled by the pumping-up rate but is materially independent from the opening degree of the guide vanes. FIG. 4 is intended for explanation of the fact that the pump input is drastically increased in accordance with the increase in the guide vane opening GVO as explained in FIG. 3. The points B, C, D and E which are plotted on the pump input curve and the guide vane opening curve in FIG. 3, as well as on the input curve in FIG. 4, respectively correspond to points B, C, D and E on the load curve shown in FIG. 4.
When the pump is started at constant speed No and constant head Ho as in the conventional method, the operation point of the pump is progressively moved from the point A corresponding to the fully closed state of the guide vanes to the normal steady operation E where the guide vane opening GVO is 100%, past the points B, C and D at which the guide vane opening is 40%, 60% and 80%, respectively, along the load curve shown in FIG. 4. The term "load curve" in this case is used to mean a curve which represents the relationship between the delivery head and the flow rate obtained when the water level is assumed to be constant taking into account the loss of head along the penstock.
FIG. 5 shows the changes in the pump input during starting of the pump turbine along the load curve from the point A to the point E for each of a high, medium and low delivery heads. It will be seen that the pump input is drastically increased as the guide vanes are opened, however the guide vane opening GVO may be small.
On the other hand, in the operation range in which the guide vane opening is extremely small, the operation is unstable due to pulsation of the water pressure and consequent vibration of the mechanical parts. From the view point of durability, therefore, it is not preferred to maintain the pump turbine in this state for a long time. In the conventional starting method, the pump is started at a constant speed of the pump turbine, so that the pump input tends to increase immediately after the start of the pumping up operation as shown in FIG. 3, causing a heavy impact and influence on the electric power line. This problem cannot be overcome insofar as the pump turbine is started at a constant speed because of the facts that the pump input is considerably large even when the guide vanes are fully closed, that the pump input is rather insensitive to the guide vane opening after reaching steady operation (FIG. 5 and point E in FIGS. 3 and 4) and that the operation is unstable when the guide vane opening is extremely small.
FIG. 6 shows the operation characteristics of a variable-speed pump in which the axis of abscissa represents the flow rate, while the axis of ordinate represents the input and the head in the form of envelope curves. In this case, however, the guide vane opening curve is omitted for the purpose of clarification of the Figure, and the characteristics as obtained with the guide vane opening which provides the optimum operating condition, i.e., the maximum efficiency, are shown by the envelope curves. Thus, characteristics for different guide vane openings are obtainable in the same way as in FIG. 4, although such characteristics are not shown in FIG. 6. In FIG. 6, the operation speed No is the operation speed during steady operation, necessary for attaining the desired flow rate Qo and the input Po at a given delivery head Ho. On the other hand, N.sub.1 is the stable starting speed at which the downward convexity of the QH curve takes a value slightly above the load curve obtained with the static head Ho, and N.sub.2 is the minimal starting speed at which the delivery head with the guide vanes fully closed takes a value slightly above the head Ho.
In starting the pump along the load curve corresponding to the delivery head Ho, if the guide vanes are opened after a guide-vane full close operation at a pump speed below the minimal starting speed N.sub.2, the water flows backwardly into the pump because the static head Ho is higher than the pump delivery pressure.
If the guide vanes are opened after the guidevane full close operation at the minimal starting speed N.sub.2, the pump operation cannot be shifted to the normal steady operation Z if this speed is maintained. For shifting the operation to the point Z, it is necessary to increase the pump speed without delay. In such a case, the point of operation is moved from X.sub.2 to Z past Y, whereas the input is changed from a point J.sub.2 to a point L past a point near a point K.
If the guide vanes are opened after guide-vane full close operation at the stable starting speed N.sub.1, the pump operation speed is automatically moved from the guidevane full close position X.sub.1 to the stable operation point Y along the load curve, without requiring acceleration from the speed N.sub.1. Then, the operation point is further shifted to the point Z in accordance with an increase in the pump speed. Meanwhile, the pump input is shifted from the point J.sub.1 to the point L past the point K.
In the conventional starting method, the pump operates at the speed No which corresponds to a head Xo which is higher than the point X.sub.1 or X.sub.2 which in turn are higher than the head Ho. When the guide vanes are opened in this state, the pump input is increased from the point Jo to the point L. As stated above, the rate of increase in the pump input can be reduced if the pumping of water is commenced at the speed N.sub.2 or N.sub.1. Unfortunately, however, the conventional method starts the pumping operation by opening the guide vanes while the pump is operating at a constant high speed. With this conventional method, therefore, the pump input is inevitably increased drastically when the pumping is commenced, causing unfavourable effect on the electric power line.